In recent years, it has become desirable for printed wiring boards to include a substrate with good heat dissipation properties, as electronic components are mounted in higher density. A metal core substrate has been known as a printed wiring board excellent in heat dissipation properties, and has already been put to practical use. The metal core substrate uses a metal having high thermal conductivity as a core material, and thereby can dissipate heat from a heat-generating component throughout the printed wiring board and suppress an increase in the temperature of the heat-generating component. Above all, aluminum, which has a low specific gravity, is generally used as a core material.
However, aluminum has a high thermal expansion coefficient of about 24 ppm/K, whereas a ceramic component has a low thermal expansion coefficient of about 7 ppm/K. Therefore, there arises a problem that when a heat cycle test is conducted, a crack occurs at a solder joint portion due to a difference in thermal expansion coefficient between aluminum and the ceramic component, failing to achieve mounting reliability.
As a core material capable of solving the above problem, carbon fiber reinforced plastics (hereinafter also referred to as CFRPs) have been known (for example, see Japanese Patent Laying-Open No. 11-40902 (Patent Literature 1)). A CFRP is a composite material made of carbon fiber and a resin. The carbon fiber has a low thermal expansion coefficient of ±2 ppm/K. Some of carbon fibers have a thermal conductivity of not less than 500 Wm−1K−1. In addition, the carbon fiber has a low specific gravity of about 2 g/cm3. If a core substrate can be fabricated using a CFRP, a substrate with high thermal conductivity and more excellent in mounting reliability than aluminum can be obtained. The CFRP has an in-plane thermal expansion coefficient of about 0 ppm/K, and an out-of-plane thermal expansion coefficient of about 30 ppm/K.
Since the CFRP is electrically conductive, the CFRP is insulated from a penetrating through hole for connecting wirings above and below a core. After the CFRP is insulated from the penetrating through hole, a predetermined circuit is formed on at least one of a front surface and a back surface of the core. After the circuit is formed, a predetermined component is mounted.